Tartan Tooling is a technique of consolidating metal powders to
produce complex cavities and cores for plastics injection molding. The
consolidation technique traditionally required an initial master from
which duplicates could be rapidly generated; until recently, duplicates
were made only in a composite material consisting of stainless Stellite
(Stoody Deloro Stellite, Inc.) and copper alloy. However, ongoing R
& D has led to a new type A6 tool steel-base composite. This article
reviews the Tartan Tool process and presents both Tartan composite
materials.

Process

The process is initiated with a female master or model, shown in Fig.
1. To compensate for Tartan Tooling process shrinkage, the master is
oversized by 0.8%, or 0.008 in/in, and its parting line and external
surfaces contain additional 0.010-in stock, which acts as grind stock
for final fitting of the inserts to the mold base. The master may be
composed of any rigid material, such as aluminum, wood, or plastic.

A variant to master preparation is shown in Fig. 2 - a male,
prepared in the shape of a piece part, is fitted to a metal chase or
enclosure to define the outside dimensions of the cavity; these
dimensions must include the 0.010-in grind stock mentioned above. This
master will be used to cast a precision urethane reverse that will serve
as the master depicted in Fig. 1. The casting process has required that
male masters be made of metal; they must also be designed 1.0% oversize because the reverse shrinks by 0.2%.

Preparing a proprietary mold from the master is the next step in
the process. This mold differs from the complex punch and die set that
is used for conventional powder metallurgy in that it is substantially
less expensive and can be rapidly produced. It captures with high
fidelity the dimensions of the master, which must be carefully machined
and inspected because each insert will contain any flaws inadvertently
left on the master. Because the master is not subject to heat or
pressure during moldmaking, it does not experience the degree of wear
that it would in pressure casting or hobbing.

Next, the mold is used to create powder metal inserts, which are
sintered in computer-controlled furnaces and combined with a copper
alloy. Four inserts can be produced in three weeks; 20 can be produced
in four weeks. Starting in the fifth week, larger quantities can be
produced at the rate of 30/week. The tolerance of the inserts is [+ or
-] 0.001 in/in.

The best surface finish of the inserts is 20 to 25 microinches. The
master need not be polished beyond this finish; in fact, a large
proportion of the cavities produced to date are not polished beyond this
finish, which is quite adequate for many engineering applications. When
a superior finish is required, conventional polishing can produce a
finish rated 2 or 3 in the SPI Mold Finish Guide.

Because of the difficulty of properly filling the deep, narrow
channels of the proprietary mold, length-to-diameter ratios greater than
4:1 are often difficult to mold. Barring this restriction, virtually any
detail placed on the master may be duplicated with high fidelity on each
cavity.

Stellite/Copper-Tin Alloy

The material originally used in the process was a cobalt-base
Stellite material combined with a copper-tin alloy. A sketch of the
material's microstructure is shown in Fig. 3. Approximately 70% of
this metallic composite consists of Stellite, an alloy well known in the
metals industry for its outstanding resistance to corrosion and wear.
Stellite comprises cobalt, chromium, tungsten, and carbon, which
represent 35%, 21%, 9%, and 1.5%, respectively, of the composite's
chemical composition. Dispersed within its cobalt-base solid solution
are fine carbides that enhance a hard, Rockwell C50 matrix.

The remaining 30% of the composite consists of a copper-tin alloy,
which increases the material's toughness and thermal conductivity.
The material's thermal conductivity is similar to that of
conventionally machined H-13 die material.

Although Stellite alone exhibits a hardness of Rockwell C55, its
combination with a less hard copper alloy produces a composite hardness
of C41-43. Field experience indicates that the material's cavity
life is similar to that of H-13 material treated to C50-52. This
suggests that the Stellite component dictates performance in this
application. Further substantiation is provided by laboratory controlled
sand abrasion tests, in which the composite exhibits wear resistance
similar to that of 440C martensitic stainless steel. A summary of the
composite's properties appears in Table 1. [Tabular Data Omitted]

A less obvious advantage of the material is that though its
corrosion resistance is similar to that of stainless steel, its thermal
conductivity is, as indicated above, similar to that of H-13 tool steel.
Thus, because of the relationship of the mold material's thermal
conductivity to cycle time, the material can be used to mold corrosive
plastics without longer cycle times that are caused by stainless
steel's lower thermal conductivity.

The Stellite/copper alloy material is supplied at a hardness of
C41-43, with cavity detail to 0.001 in/in. Its proper fit in the mold
base requires that its external dimensions be altered through some minor
secondary machining operations, which may be readily accomplished by
means of conventional alumina grinding wheels such as A60JV. Because of
the Stellite/copper-tin alloy's wear-resistant nature, carbide is
recommended for milling operations.

A6 Tool Steel/Copper Alloy

The new A6 tool steel/copper alloy was developed to meet the need for
a tough, hard alternative tool steel material. It comprises type A6 tool
steel, tungsten carbide, and copper alloy, in percentages of 63.8, 2.7,
and 33.5, respectively. The base material, type A6 tool steel, is
magnetic and thus permits magnetic chuckdown; it contains 0.6% carbon,
2.0% manganese, 1.0% chromium, and 1.3% molybdenum. It was chosen
because of its deep air-hardening characteristics and tendency for low
and predictable dimensional change due to heat treatment. It is
important that little if any dimensional change and distortion occur
during hardening, because all of the detail is molded into these
cavities. Vacuum heat treatment, followed by gas quenching, has been
found to be the optimum austenitizing treatment.

The A6 tool steel contains a tungsten carbide dispersion. Fine (1-5
microns in diameter) and uniformly spaced, the carbides add wear
resistance to the A6 matrix, but they do not significantly affect
secondary machining operations.

The final component of the material is the copper alloy, a
continuous or interconnected phase that confers substantially improved
heat transfer properties. Tests conducted at the Thermotest Division of
Holometrix, Inc., of Cambridge, Mass., have revealed, within the
temperature range of 43 [degrees] C to 343 [degrees] C, thermal
conductivities in excess of 50% greater than those of conventional H-13
tool steel. Hence, cycle-time reductions are also evident.

Type A6-base composite cavities are supplied at three hardness
levels: Rockwells C30-32, C40-42, and C48-50. The C30-32 level is
selected when a number of secondary machining operations, such as
extensive drilling and tapping, are to be performed. Subsequent to such
machining, cavities may be heat-treated to C48-50 with minimal
distortion. The C40-42 range is intended for long-run applications that
require high toughness and good wear resistance. At this level, the
cavities may be conventionally ground and carbide taps may be used.

The Rockwell C48-50 temper is used for long-run applications that
require maximum wear resistance and good peening resistance. When
supplied in this condition, the material may be conventionally ground,
but it cannot be conventionally tapped. A carbide cutter may be used to
mill the material.

Cycle Time Testing

Substantial improvement in thermal conductivity of the Tartan
materials, in comparison to that of conventional die materials,
motivated an evaluation of cycle time. It was reasoned that reducing
cycle time with the use of Tartan Tooling could further improve the
economics of molding and the quality of plastic parts. A field
evaluation was conducted at Contract Design, Inc., of Minneapolis, to
compare Tartan tool A6 to conventional H-13 and 420 stainless steels and
Tartan stainless Stellite material.

Initially, testing was conducted with the use of a 75-ton Nissei P
NC8000 injection molding machine. However, because the Tartan A6's
rapid cycle time appeared to push the 75-ton machine to its limit, the
tool was moved to a 100-ton JSW.A1-1/2-inch-diameter disk in thicknesses
of 0.040,0.080, and 0.120 inch was used; two types of plastics,
polypropylene (PP) and GE Noryl, were run.

The following responses were used to judge cycle time: complete
fill, freedom from ejector pin punch, and freedom from doming or
cupping. Cycle time was adjusted downward until any criterion was
violated; the amount of time up to the point just prior to any criterion
violation was defined as the minimum cycle time.

Cycle times for the two tests ranged from 5.95 seconds for the
0.040-in Noryl disk molded in Tartan A6 to 30.9 seconds for the 0.120-in
PP disk molded in 420 stainless steel. The results of the two tests are
shown in Table 2.

Computing averages over thicknesses and plastic types yields the
following findings:

1. The Tartan Tool A6 composite cavities showed significant cycle
time improvements in relation to H-13 and 420 stainless steel cavities,
the cycle times of which were 13.3% and 22% longer, respectively, than
those of A6.

2. Although the Tartan Tool A6 had shorter cycle times than those
of Tartan Stellite/copper alloy material, the difference averaged only
4%. This suggests that in the heat transfer process, the copper alloy is
dominant.

3. The Tartan stainless Stellite/copper alloy material had cycle
times averaging 18% shorter than those of 420 and 9.5% shorter than
those of H-13. Therefore, in applications that require corrosion
resistance comparable to that of stainless steel, the Stellite/copper
alloy material provides much more rapid cycle times.

Further analysis involved the use of Moldflow's Moldtemp
software to cool the mold with different mold materials. It was found
that with regard to H-13 tool steel and stainless steel, attempts to
eject the parts at the Tartan A6 cycle time resulted in much hotter
steel temperatures and larger temperature gradients.

According to the software, to completely freeze PP parts down to
the vicat softening point (144 [degrees] C for the material used) takes
0.5 seconds longer with H-13, and 1.6 seconds longer with stainless
steel, than it does with A6. Even at these longer times, however, the
steel temperature gradient on the parts is still much greater than it is
with A6. Large variations in mold surface temperature are undesirable;
they are likely to cause uneven cooling of the parts and, hence,
differential shrinkage. Thus, even with the longer cycle times of H-13
and 420, equivalent part quality will not be achieved.

PHOTO : FIGURE 1. A female master, the generation of which is the
first step in cavity-making.

PHOTO : FIGURE 2. A male master, which is prepared in the shape of
a piece part and fitted to a metal chase or enclosure.